1. Field
The present invention relates to field programmable gate array (FPGA) circuits. More specifically, the present invention relates to a circuit which can automatically determine the starting point (or seed) for a background noise estimator (BNE) circuit of an FPGA circuit.
2. Related Art
Most known receivers use a Constant False Alarm Rate (CFAR) circuit to detect signals. The CFAR threshold is set to be a certain decibel level above the noise floor which is typically tracked with a Background Noise Estimator (BNE) circuit. A signal is detected when the magnitude level rises above the CFAR threshold and measurements or demodulation are then triggered. The BNE circuit is prevented from adjusting during a detection so that the estimated noise floor will not adjust upwards to the detected signal level.
A problem with BNE circuits is that they typically lose track of the noise floor when the receiver front end is tuned to a different frequency band. This problem is due to switching noise, local oscillator settling times, large jumps in the noise floor level, and tuning into a large continuous wave (CW) signal. This can cause a significant increase in false alarms or failed detections even though a signal is present.
A solution to this problem is to use a manual seed (starting point for tracking noise floor) that is input into the BNE to restart the BNE at a particular noise floor level. Currently, this is done manually by adjusting fixed thresholds until an acceptable false alarm rate is reached. Then the BNE seed is selected to be about 3 dB below that point. Another solution is to have a pre-determined BNE seed that is a typical value for each band. The first method is very cumbersome and time consuming, and the second method does not always work in real operating conditions so it can also have the problem of increased false alarms or failed detections. The BNE seed may be picked too high, picked correctly, or too low.
In FIG. 1, the BNE seed has been picked too low. The receiver will instantly start a detection since the BNE does not change during a detection. The receiver will collect data throughout the collection period (or dwell) even though only noise is present.
In FIG. 2, the BNE seed is picked high. In this case, since there is a pulse noise present, the BNE will track down to the noise floor and good detection will result. The BNE would also track in this scenario even if only noise would have been present. However, because the BNE seed is picked high, it takes more time for the BNE to track down to the noise floor.
In FIG. 3, the BNE seed is once again picked too high, but there is a continuous wave (CW) signal present in the band. The BNE will track down and adjust to the magnitude of the CW signal as if it were the noise floor. No CW signal will be detected, thus no data will be collected.
Accordingly, there is a need for an improved method of determining a BNE seed value that does not suffer from the problems and limitations of the prior art.